1. Field of the Invention
The present invention relates to a method for manufacturing a laser module that is constituted with a planar lightwave circuit and an optical semiconductor element, and to the laser module.
2. Description of the Related Art
As a laser module constituted with an optical semiconductor element and a planar lightwave circuit or an optical fiber instead of the planar lightwave circuit and, “Coupling Structure of Optical Semiconductor Element and Optical Transmission Path” disclosed in Japanese Unexamined Patent Publication 2000-231041 (paragraph 0015—paragraph 0016, FIG. 1) (Patent Document 1), “Optical Module” disclosed in Japanese Unexamined Patent Publication 2005-17388 (paragraph 0048—paragraph 0052, FIG. 1, FIG. 2) (Patent Document 2), and “Optical Transmitter/Receiver” disclosed in Japanese Unexamined Patent Publication Hei 8-304671 (paragraph 0011, FIG. 2, FIG. 4) (Patent Document 3), for example, are already known. The combined structure disclosed in Patent Document 1 is as follows. That is, an electrode pad is provided on a mount face that is formed on a planar lightwave circuit by notching a part of a planar lightwave circuit constituting a part of a laser module from a side of a waveguide forming face of the planar lightwave circuit towards the normal direction, and an optical semiconductor element constituting the other part of the laser module is mounted on the electrode pad in a junction-down state via a solder layer to align the height of an active layer of the optical semiconductor element with the height of the waveguide of the planar lightwave circuit in the normal direction of the waveguide forming face.
With this structure, the optical semiconductor element is mounted on the planar lightwave circuit in a junction-down state where the active layer of the optical semiconductor element is adjacent to the mount face of the planar lightwave circuit. Thus, it is possible to match the height of the active layer of the optical semiconductor element with the height of the waveguide of the planar lightwave circuit in the normal direction of the waveguide forming face without forming a deep notch from the surface of the waveguide forming face side of the planar lightwave circuit towards the normal direction. Since a deep notch is not required and the height of the active layer of the optical semiconductor element and the height of the waveguide of the planar lightwave circuit are originally close, there is such a merit that processing errors and assembling errors are not likely to occur. Meanwhile, the stress of the solder works on the active layer of the optical semiconductor element that is very close to the mount face of the planar lightwave circuit, so that oscillation of the laser becomes unstable and the SMSR property (Sub-Mode Suppression Ratio) becomes deteriorated.
In the meantime, in the optical module disclosed in Patent Document 2, an optical semiconductor element is attached in a junction-up state on a mount face that is formed on a V-groove substrate by notching the V-groove substrate for attaching an optical fiber from the top-face side towards the normal direction. Thus, it is necessary to align the height of an active layer of the optical semiconductor element and the height of the optical fiber on the V-groove substrate by deeply notching the V-groove substrate from the top-face side towards the normal direction. However, the active layer of the optical semiconductor element becomes isolated from the mount face on the V-groove substrate for the amount corresponding to the thickness of the optical semiconductor element, so that it is possible to overcome such inconvenience that the stress of a solder layer works directly on the active layer of the optical semiconductor element. This makes it possible to improve the SMSR property by stabilizing the oscillation of the laser.
With the optical module disclosed in Patent Document 2, in order to prevent redundancy of processing errors generated by deeply notching the V-groove substrate from the top-face side towards the normal direction and variation in the thickness of the optical semiconductor element, i.e., assembling errors generated depending on the variation in the isolated distance between the mount face and the active layer at last, a solder layer is interposed between the mount face of the V-groove substrate and the optical semiconductor element and the solder layer is crushed by pressing the surface of the active layer side of the optical semiconductor element on the surface of the mounting substrate until a surface of a mounting substrate (jig) that handles the optical semiconductor layer comes to abut against the surface on the active layer side of the optical semiconductor element and the top face (non-mount face) of the V-groove substrate simultaneously. Thereby, the optical semiconductor element is mounted to the mount face on the V-groove substrate in such a manner that the surface on the active layer side of the optical semiconductor element and the top face of the V-groove are on a same plane.
However, the heights actually need to be aligned are the height of the active layer of the optical semiconductor element and the height of the optical fiber on the V-groove substrate. Thus, it is not necessarily so important to have the surface on the active layer side of the optical semiconductor element and the top face of the V-groove on a same plane. That is, the optical fiber is attached to the V-groove of the V-groove substrate in a step after the optical semiconductor element is mounted on the V-groove substrate (see paragraph 0052 of Patent Document 2), so that the relative positional relation of the height of the active layer of the optical semiconductor element and the height of the optical fiber on the V-groove substrate cannot be guaranteed unless the optical fiber is attached properly to the V-groove substrate.
Therefore, in the optical module disclosed in Patent Document 2, there still remains such an inconvenience that the height of the active layer of the optical semiconductor element and the height of the optical fiber on the V-groove substrate may not be aligned with each other. Patent Document 3 discloses a point that an optical semiconductor element is mounted in a junction-up state on a mount face formed by etching a substrate that forms a waveguide. However, the technical significance of mounting the optical semiconductor element in a junction-up state is not specifically disclosed therein.
It is therefore an exemplary object of the present invention to provide a laser module manufacturing method which can improve the SMSR property by stabilizing the oscillation of the laser and can precisely align the height of the active layer of the optical semiconductor element with the height of the waveguide of the planar lightwave circuit in the normal direction of the waveguide forming face, and to provide a laser module.